That's Edward Hopper's most famous work, Nighthawks. His art was a blend of (at the time) modern scenery, mixed with themes of tenseness, stress, melancholy, etc. The door missing is just one of many subtle clues as to the tone of the piece as a whole-- the emptiness of the street outside, the isolation of the man on the far left, the tense body language and gaze of the other suited man, the despondent down-turn expression on the woman's face. It poses the viewer not as a person viewing the painting, but as a person viewing the tense and lonesome scene from across the street. The unease of his work comes through in every shade and stroke. I absolutely love that. It's part of why he's one of my favorite American artists.